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6 .5. The Possibility for Improved Technology and Increased EROI for Cellulosic Ethanol
The following calculations are intended to illustrate the potential for improvements in cellulosic
ethanol’s EROI. These calculations assume that the Schmer et al. [23] and Heller et al. [24] papers are
essentially correct in their estimates of the crop production phase energy inputs and that Dale’s
coauthored paper [26] provides reasonable estimates of the overall energy efficiency of converting
biomass to ethanol and electricity, given different conversion technologies.
Dale develops his argument as: “As we have seen from several different sources, by far the
dominant energy inputs to agricultural production for both corn and cellulosic biomass are in the
nitrogen fertilizer applied and also the diesel fuel used for transport and field operations. Reducing
these inputs would therefore increase the EROI for biofuels. Better fertilization practices (slow release
fertilizer, precision agriculture), use of leguminous (nitrogen fixing) crops, breeding and genetic
modification to reduce fertilizer nitrogen requirements and application of biosolids from waste water
treatment instead of synthetic nitrogen fertilizer are all methods by which fertilizer nitrogen inputs
might be reduced over time for bioenergy crops such as switchgrass and willow”.
Assuming that a future cellulosic ethanol industry is supplied with both switchgrass and willow
feedstocks in equal amounts, and that the nitrogen fertilizer inputs for these two materials would be
reduced by half from the values given in Table 3 , the total nitrogen input would be about 0.33 MJ/L of
ethanol. Also, bioenergy crops such as switchgrass and willow are in the very early stages of breeding
to increase yields with lower inputs per unit of yield, as has been done so successfully for corn and
other crops For example, fertilizer nitrogen use per bushel of corn has decreased by about one third
from 1970 through 2005 [45,46].
Dale believes that significant yield gains and more favorable nitrogen use efficiency can also be
expected for cellulosic biomass crops. For example, in 2002 in the Midwestern US, switchgrass
required about 120 kg of nitrogen (N) per ha to produce 10.2–12.6 Mg of dry biomass per ha [47]. This
is roughly equivalent to 35 MJ of switchgrass produced per MJ of fertilizer N applied (assuming 18
MJ per kg of switchgrass (lower heating value) and 48.2 MJ required to produce 1 kg of N (also lower
heating value). The energy requirements of N fertilizer production are based on recent data from the
GREET model maintained by Argonne National Laboratory (GREET 1.8d).
In contrast, in 2009, in eastern Tennessee 67 kg of N were required to yield between 15.6–22.9 MG
of dry switchgrass per ha on moderately to well drained soils, or around 108 MJ switchgrass produced
per MJ of fertilizer N, an increase of about 3 fold versus the earlier Midwestern results of Schmer, et al
[23]. Obviously, soil type, cultivar and climate all play a role in yield and nitrogen use efficiency, but
the point is that very favorable yields and nitrogen use efficiencies leading to potentially high EROI
values have already been shown for cellulosic biomass crops. Other increases in efficiency appear
possible in agricultural fuel use [49] (and also in the operation of a biorefinery [26]. Table 4 gives
Dale’s estimates for the improvements in yield and reductions in energy costs for producing
switchgrass. If all of these improvements in efficiency are realizable, as Dale thinks possible, then
EROI for cellulosic ethanol from switchgrass might be doubled from 17:1 to 35:1. If the thermal
efficiency of the biorefinery is increased (e.g., by ethanol and more net electricity produced in a gas
turbine combined cycle (GTCC) system [26], then further increases in cellulosic ethanol EROI can be
expected.